The present invention relates to a variable displacement type compressor that has a coupling mechanism for coupling a cam plate, which drives pistons, to a drive shaft and changes the reciprocation stroke of the pistons by altering the inclination angle of the cam plate by controlling the pressure in a crank chamber.
FIG. 15 shows one type of a variable displacement type compressor for use in a vehicle air-conditioning system. Accommodated in a housing 101 of the compressor are a crank chamber 102, a suction chamber 108, a discharge chamber 109 and a plurality of cylinder bores 107 (only one shown). A piston 110 is retained in each cylinder bore 107. A drive shaft 103 and a lug plate 104, which are fixed to each other, are located in the crank chamber 102. To seal the crank chamber 102, the housing 101 is provided with a lip seal 114 around the front end of the drive shaft 103. The front end of the drive shaft 103 is coupled to the engine (external drive source) of the vehicle directly or indirectly. A spring 112 for urging the drive shaft 103 in a forward direction is located at the rear end of the drive shaft 103. The spring 112 positions the drive shaft 103 and the lug plate 104 in the crank chamber 102 in the axial direction while absorbing the tolerances of the drive shaft 103 and various components associated with the drive shaft 103.
Provided around the drive shaft 103 is a swash plate 105, or cam plate. The swash plate 105, which is coupled to the individual pistons 110 via shoes 113, converts the rotational motion of the drive shaft 103 to reciprocal motion of each piston 110. This swash plate 105 is coupled to the lug plate (rotary support) 104 via a coupling mechanism 115. The coupling mechanism 115 has guide pins 116 protruding from the front face of the swash plate 105 and support arms 117 protruding from the rear face of the lug plate 104. The head of each guide pin 116 is inserted into a cylindrical guide hole 117a formed in the associated support arm 117. This coupling mechanism 115 allows the swash plate 105 to rotate with the drive shaft 103 and to tilt as the swash plate 105 moves along the drive shaft 103 (in the axial direction).
The stroke of the pistons 110, or the discharge displacement, is determined by the inclination angle of the swash plate 105, which is mainly determined by the difference between the pressure of the crank chamber 102 (crank pressure Pc) and the pressure in the cylinder bores 107 via the associated piston 110. This difference is controlled by a displacement control valve 120. Generally speaking, as the crank pressure Pc rises, the swash plate 105 disinclines, or slides on the drive shaft 103 away from the lug plate 104, making the inclination angle of the swash plate 105 smaller. A restriction ring 106 is fixed on the drive shaft 103 so that, when the swash plate 105 contacts the restriction ring 106, further disinclination of the swash plate 105 is restricted, thereby defining the minimum inclination angle of the swash plate 105. In the compressor in FIG. 15, the control mechanism for the crank pressure Pc comprises a restriction-equipped bleed passage 118, which connects the crank chamber 102 to the suction chamber 108, an supply passage 119, which connects the discharge chamber 109 to the crank chamber 102, and the displacement control valve 120 located midway in the supply passage 119. The opening of this displacement control valve 120 can be adjusted by external energization. As the opening of this control valve 120 is adjusted externally, the amount of high-pressure refrigerant gas supplied into the crank chamber 102 from the discharge chamber 109 via the supply passage 119 is adjusted. The crank pressure Pc is determined by the relationship between the flow rate of gas supplied to the crank chamber 102 and the flow rate of gas that is released from the crank chamber 102 via the bleed passage 118.
In the air-conditioning system of a vehicle, the capacity of the compressor is minimized to reduce the engine load as much as possible when rapidly accelerating the vehicle. When the air-conditioning system is switched off or the engine is stopped, the discharge capacity of the compressor is often minimized in advance to prevent the next activation of the compressor from applying an excess load to the engine. As far as the compressor in FIG. 15 is concerned, the capacity of the compressor is minimized by supplying high-pressure refrigerant gas into the crank chamber 102 from the discharge chamber 109 with the displacement control valve fully opened by an external signal. To minimize the capacity of the compressor when rapidly accelerating the vehicle, particularly, it is necessary to quickly minimize the discharge capacity. Thus, high-pressure refrigerant gas is often rapidly led into the crank chamber 102.
When high-pressure gas in the discharge chamber 109 is led into the crank chamber 102 to swiftly increase the crank pressure Pc, however, various problems may arise depending on the amount of the pressure rise. Anything serious may not occur until the sudden rise of the crank pressure Pc minimizes the inclination angle of the swash plate 105. If the difference between the crank pressure and the cylinder-bore inner pressure is too large even after the inclination angle of the swash plate 105 is minimized, the excess pressure difference causes the pistons 110 to move rearward (in the direction away from the lug plate). This applies a rearward force to the swash plate 105. At this time, the inclination angle of the swash plate 105 is minimized and the swash plate 105 abuts against the restriction ring 106. When the rearward force acts on the swash plate 105, therefore, the swash plate 105 urges the drive shaft 103 against the force of the spring 112 via the restriction ring 106. Further, the swash plate 105 is coupled to the lug plate 104 by the engagement of each guide pin 116 and the associated guide hole 117a of the coupling mechanism 115. If the swash plate 105 is rapidly disinclined, the swash plate 105 pulls the lug plate 104 and the drive shaft 103 rearward against the force of the spring 112. In other words, when the crank pressure becomes too large, a strong rearward force acts on the entire inner mechanism of the compressor which includes the pistons, the swash plate, the coupling mechanism, the lug plate and the drive shaft, causing those components to move rearward beyond the design limit for such movement(i.e., the axial position corresponding to the minimum inclination angle of the swash plate 105). This brings about the following problems.
Problem 1: When the drive shaft 103 moves rearward beyond the design limit, the position of contact between the lip seal 114 and the drive shaft 103 changes from a predetermined position called the contact line. Foreign matter such as sludge adheres to the outer surface of the drive shaft 103 at locations other than the contact line. If the drive shaft 103 moves axially, therefore, foreign matter may come between the outer surface of the drive shaft 103 and the lip seal 114, which will break the seal produced by the lip seal 114.
Problem 2: In some compressors of vehicles, an electromagnetic clutch is located in the power transmitting path between the engine and the drive shaft 103. The typical electromagnetic clutch has a drive clutch plate on the engine side and a driven clutch plate (armature), which rotates with the drive shaft 103 and can be shifted axially by the force of a spring. The clutch is engaged by electromagnetically engaging the armature and the drive clutch plate when the electric power is cut off, a predetermined gap should exist between the armature and the drive clutch plate. When the engine is stopped, in the air-conditioning system, the electromagnetic clutch is deactivated and the displacement control valve 120 is fully opened. As the displacement control valve 120 is fully open, as mentioned above, the drive shaft 103 moves further rearward beyond the design limit. Despite the power cutoff, therefore, the armature together with the drive shaft 103 approaches the drive clutch plate from the original separated position so that the predetermined gap between both clutch plates may not be secured at all. That is, in spite of the attempted power cutoff action, the armature and the drive clutch plate have a slide contact with each other. This slide contact not only disables the power cutoff but also brings about a new problem of producing noise or heat or wearing the clutch plates.